An ink jet printing system performs printing by ejecting an ink liquid from a nozzle towards a printing medium using a pressure, heat or an electric field as a driving source. Such a printing system has low running cost and achieves high printing quality, and it can use various inks such as aqueous inks, oil-based inks, etc. Therefore, the ink jet printing system extends the market in these years.
Nowadays, an image-forming method by the ink jet printing system has great advantages from an economical standpoint in applications which frequently change printed materials such as large size printed advertisements that require a large printing area, since it can provide the same image quality as silver halide photography at very reasonable cost.
As ink compositions used in the ink jet printing system, ink compositions which are cured by the irradiation of an active energy ray in a specific wavelength range, in particular, UV-curable ink compositions which are cured by the irradiation of ultraviolet (UV) ray are available. The major components of the UV-curable ink compositions include UV-curable monomers, pigments or dyes as colorants, and photopolymerization initiators. If necessary, the ink compositions may optionally contain various additives.
However, the viscosity of the UV-curable ink compositions usually exceeds 35 mPa·s at 25° C. When the UV-curable ink compositions are used in the existing ink jet printing system, their viscosity should be 35 mPa·s or less, preferably 20 mPa·s or less.
To achieve such a low viscosity, a UV-curable ink composition containing a volatile organic solvent such as ethyl lactate, butyl acetate, etc. as a diluent solvent together with a UV-curable monomer, a colorant and a photopolymerization initiator is known. However, when such an ink composition is used, the evaporation of the diluent solvent such as VOC (volatile organic compound) may worsen work environment and natural environment. In addition, the diluent solvent remains in the printed ink after the composition is cured with UV ray, so that the film properties of the printed images may decrease, in particular, the adhesion of the printed images to a substrate of a printing medium may decrease.
A UV-curable ink composition is also proposed, which ink composition comprises a vinyl compound, an alkyl (meth)acrylate, a polyfunctional (poly)alkoxyacrylate, etc. as a UV-curable monomer and contains the decreased amount of a diluent solvent (JP-A-5-214279, JP-A-5-214280 and JP-A-2000-504778).
However, the proposed UV-curable ink compositions usually have low monomer reactivity and thus it cannot have a sufficient curing property, since oxygen interferes with the curing at a low energy level such as a cumulative amount of light of 150 mJ/cm2 or less, and therefore such ink compositions may have limitations in high-speed printing applications.
The curing property increases as the amount of the photopolymerization initiator increases. However, the solubility of the photopolymerization initiator in the monomer is limited, and thus the amount of the photopolymerization initiator compounded in the ink composition is limited and insufficient for attaining the sufficient curing property. In addition, when a large amount of the photopolymerization initiator is compounded, the unreacted photopolymerization initiator remains and tends to have adverse effects on the properties of the cured film.